Pretreatment for electroplating process

ABSTRACT

A method for metal plating the surface of an article formed from an organic plastic. The method includes a step of passing a current between two electrodes immersed in an electrolyte containing dissolved plating metal. One of the electrodes is the article to be plated and is provided with a surface having areas of a catalytic metal chalcogenide conversion coating adjacent to and in contact with conductive areas. In a preferred embodiment, the catalytic metal chalcogenide conversion coating is formed by treating the article with an acid colloidal solution of a tin-noble metal electroless metal plating catalyst and, subsequently, treating with a solution containing a dissolved sulfide to form a sulfide of the noble metal. The conversion coating allows the article to be directly electroplated. The method is especially useful for the formation of printed circuit boards and is sufficiently versatile to permit formation of a printed circuit board by a process that involves pattern plating.

Cross Reference to Related Applications

This is a divisional of co-pending application Ser. No. 07/153,357 filedon Feb. 8, 1988 which is a continuation in part of copending U.S. Pat.Application Ser. No. 071,865 filed July 10, 1987 abandoned.

Background of the Invention

I. Introduction.

This invention relates to electroplating nonconductors, and moreparticularly, to a process for electroplating the surface of anonconductor by converting an adsorbed colloid into a chemicallyresistant, metal chalcogenide conversion coating, which functions as abase for direct electroplating. The invention is particularly useful inthe manufacture of printed circuit boards where copper is deposited overa circuit board base material.

II. Description of the Prior Art.

Nonconductive surfaces are conventionally metalized by a sequence ofsteps comprising catalysis of the surface of the nonconductor to renderthe same catalytic to an electroless metal deposit followed by contactof the catalyzed surface with an electroless plating solution thatdeposits metal over the catalyzed surface in the absence of an externalsource of electricity. Metal plating continues for a time sufficient toform a metal deposit of the desired thickness. Following electrolessmetal deposition, the electroless metal deposit is optionally enhancedby electrodeposition of a metal over the electroless metal coating to adesired full thickness. Electrolytic deposition is possible because theelectroless metal deposit serves as a conductive coating that permitselectroplating.

Catalyst compositions used for electroless metal plating are known inthe art and disclosed in numerous publications including U.S. Pat. No.3,011,920, incorporated herein by reference. The catalysts of thispatent consist of an aqueous suspension of a tin noble or precious(catalytic) metal colloid. Surfaces treated with such catalysts promotethe generation of electrolessly formed metal deposits by the oxidationof a reducing agent in an electroless plating solution catalyzed by thecatalytic colloid.

Electroless plating solutions are aqueous solutions containing dissolvedmetal and reducing agent in solution. The presence of dissolved metaland reducing agent together in solution results in plate out of metal incontact with a catalytic metal tin catalyst. However, the presence ofthe dissolved metal and reducing agent together in solution can alsoresult in solution instability and indiscriminate deposition of metal onthe walls of containers for such plating solutions. This may necessitateinterruption of the plating operation, removal of the plating solutionfrom the tank and cleaning of tank walls and bottoms by means of anetching operation. Indiscriminate deposition may be avoided by carefulcontrol of the plating solution during use and by the use of stabilizerswhich inhibit indiscriminate deposition, but which also retard platingrate.

Attempts have been made in the past to avoid the need for an electrolessplating solution by a direct plating process whereby a metal may bedeposited directly over a treated nonconductive surface. One suchprocess is disclosed in U.S. Pat. No. 3,099,608, incorporated herein byreference. The process disclosed in this patent involves treatment ofthe nonconductive surface with a tin palladium colloid which forms anessentially nonconductive film of colloidal palladium particles over thenonconductive surface. This is the same tin palladium colloid used as aplating catalyst for electroless metal deposition. For reasons not fullyunderstood, it is possible to electroplate directly over the catalyzedsurface of the nonconductor from an electroplating solution thoughdeposition occurs by propagation from a conductive surface. Therefore,deposition begins at the interface of a conductive surface and thecatalyzed nonconductive surface. The deposit grows epitaxially along thecatalyzed surface from this interface. For this reason, metal depositiononto the substrate using this process is slow. Moreover, depositthickness is uneven with the thickest deposit occurring at the interfacewith the conductive surface and the thinnest deposit occurring at apoint most remote from said interface.

An improvement in the process of U.S. Pat. No. 3,099,608 is disclosed inU.K. Patent No. 2,123,036B, incorporated herein by reference. Inaccordance with the process described in this patent, a surface isprovided with metallic sites and the surface is then electroplated froman electroplating solution containing an additive that is said toinhibit deposition of metal on the metal surface formed by platingwithout inhibiting deposition on the metallic sites over thenonconductive surface. In this way, there is said to be preferentialdeposition over the metallic sites with a concomitant increase in theoverall plating rate. In accordance with the patent, the metallic sitesare preferably formed in the same manner as in the aforesaid U.S. Pat.No. 3,099,608--i.e., by immersion of the nonconductive surface in asolution of a tin palladium colloid. The additive in the electroplatingsolution responsible for inhibiting deposition is described as oneselected from the group of dyes, surfactants, chelating agents,brighteners and leveling agents. Many of such materials are conventionaladditives for electroplating solutions.

There are limitations to the above process. Both the processes of theU.S. and the U.K. patents for direct electroplating require conductivesurfaces for initiation and propagation of the electroplated metaldeposit. For this reason, the processes are limited in their applicationto metal plating of nonconductive substrates in areas in close proximityto a conductive surface.

One commercial application of the process of the U.K. patent is themetallization of the walls of through holes in the manufacture ofdouble-sided printed circuit boards by a process known in the art aspanel plating. In this application, the starting material is a printedcircuit board substrate clad on both of its surfaces with copper. Holesare drilled through the printed circuit substrate at desired locations.For conductivity, the hole walls are catalyzed with a tin palladiumcolloid to form the required metallic sites on the surfaces of the wallsof the through holes. Since the circuit board material is clad on bothof its surfaces with copper and the circuit board base material is oflimited thickness, the copper cladding on the surfaces of the circuitboard material is separated by the thin cross section of the substratematerial. The next step in the process is direct electroplating over thecatalyzed hole walls. Since the copper cladding on each surface isseparated by the cross section of the substrate, during electroplating,deposition initiates at the interfaces of the copper cladding and thethrough hole walls and rapidly propagates into the holes. The wall isplated to desired thickness within a reasonable period of time.Thereafter, the circuit board is finished by imaging and etchingoperations.

A disadvantage to the above panel plating process is that copper iselectroplated over the hole wall and over the entire surface of thecopper cladding. The steps following plating involve imaging with anorganic coating to form a circuit pattern and removal of copper byetching. Therefore, copper is first electrolytically deposited and thenremoved by etching, a sequence of steps which is wasteful of platingmetal, etchant and time, and therefore, more expensive.

The art, recognizing the disadvantages of panel plating, has developed amethod for manufacture of printed circuit boards known as patternplating. In this process, a printed circuit board base material isdrilled at desired locations to form through holes. The through holesare metalized using conventional electroless plating techniques.Electroless copper is plated onto the walls of the through holes andover the copper cladding. Thereafter, photoresist is applied and imagedto form the circuit pattern. The board is then electroplated with copperdepositing on the copper conductors and through hole walls, but not overthe entire surface of the copper cladding. Solder mask is then platedover the exposed copper by immersion or electroplating and the remainingphotoresist is stripped. The copper not protected by the solder is thenremoved by etching to form the copper circuit.

Pattern plating cannot be used with the metalizing process of theaforesaid U.K. patent. The treatment of the copper cladding prior to theapplication of the photoresist and the development of the photoresist,all as required for pattern plating, requires the use of treatmentchemicals found to dissolve or desorb the tin palladium colloid fromhole walls. Since this occurs prior to electroplating, directelectroplating to provide conductive through holes becomes impossible.

Copending U.S. Pat. application Ser. No. 07/071,865, filed July 10, 1987and assigned to the same assignee as the subject application, provides anew method for direct electroplating of the surface of a nonconductorand to articles manufactured by said method. The process is in animprovement over the process of the above referenced U.K. Patent.

The invention disclosed in the copending application was predicated upona combination of discoveries. One discovery was that sulfide films ofmetals that function as electroless deposition catalysts may beelectroplated directly without requiring an intermediate electrolesscoating. Another discovery of the invention was that many of suchsulfide films are insoluble and unaffected by treatment chemicals usedfor plating of plastics and circuit board fabrication and therefore, theprocess of the invention was suitable for the formation of printedcircuits using pattern plating procedures.

The process of the copending patent application is illustrated by theplating sequence that follows and is compared to a conventional platingprocess requiring electroless metal deposition.

    ______________________________________                                        Conventional Process (A)                                                                          Inventive Process (B)                                     ______________________________________                                        Step 1 Desmear with chromic or                                                                        Desmear with chromic or                                      sulfuric acid or plasma                                                                        sulfuric acid or plasma                               Step 2 Clean and condition with                                                                       Clean and condition with                                     detergent type material                                                                        detergent type material                               Step 3A                                                                              Microetch copper cladding                                                                      --                                                    Step 4 Catalyst predip  Catalyst predip                                       Step 5 Catalyze with    Catalyze with                                                catalytic colloid                                                                              catalytic colloid                                     Step 6 Accelerate       Accelerate (optional)                                 Step 7 Deposit electroless                                                                            Treat with sulfide                                           metal            solution                                              Step 7B                                                                              --               Microetch copper cladding                             Step 8 Electroplate     Electroplate                                          ______________________________________                                    

A comparison of the two processes illustrated above demonstrates thatthe process disclosed in the copending application replaces the need forelectroless plating with a direct electroplating step therebyeliminating the need for a costly electroless metal plating solutionthat may be subject to stability and disposal problems. The eliminationof the electroless plating step is accomplished without an increase inthe total number of steps required for metal deposition. Further, theprocess of the invention was found to be unaffected by conventionalprocessing chemicals used for metal plating of plastics and formation ofprinted circuit boards.

In the process of the copending application illustrated above, contactof the catalytic metal on the surface of the nonconductor with a sulfidetreatment solution (Step 7) results in the formation of a metal sulfideconversion coating of the catalytic metal (the catalytic metal sulfide).The sulfide solution may be a simple aqueous solution of a water solublealkali or alkaline earth metal sulfide or a solution of a covalentlybonded sulfide such as a thiocarbonate or a dithiodiglycolate. Inaccordance with the invention of the copending application, thecatalytic metal sulfide formed by treatment with the sulfide solution isa suitable conversion coating for direct electroplating.

For the formation of printed circuit boards using the process of thecopending application, it is preferred that an etching step be usedsubsequent to formation of the catalytic metal sulfide film over thesurface of the nonconductor (Step 7B above). This etching step may usethe same etchants as used in the conventional process to clean coppercladding (Step 3A above). It is preferred that the etching step bedeferred to a point subsequent to the step of formation of the catalyticmetal sulfide conversion coating so that the etchant may remove sulfideresidues on the surface of the copper cladding. It is an advantage ofthe process that the catalytic metal sulfide conversion coating over thenonconductive surface is essentially unaffected by the step of etchingthe copper cladding. It is a further advantage that any residuesdeleterious to copper-copper bonding left on the copper by a photoresistused in the manufacture of printed circuit boards may be readily removedby a more aggressive etchant than was possible in a conventional platingline where the electroless copper is only about 100 microinches thickover the hole wall.

The final step in the process of the copending application comprisedelectroplating of the thin catalytic metal sulfide conversion coating.This was accomplished using standard electroplating procedures. Theprocedures of the above referenced U.K. Patent are suitable forelectroplating the sulfide coating described therein.

Summary of the Invention

The copending patent application was directed to a process thatpermitted direct plating over a nonconductor coated with a sulfideconversion coating. In accordance with the present invention, it hasbeen discovered that chalcogenides of colloidal particles, in additionto sulfides, function as a suitable conversion coating for directelectrolytic deposition. Consequently, in accordance with the inventionset forth herein, the range of suitable materials for formation of aconversion coating has been expanded.

Definitions

The term "non conductor" means an article having at least a portion ofits surface inadequately conductive for direct electroplating. The nonconductor must be in contact with a metal surface which can be a part ofthe surface of the non conductor or which can be placed in contact withthe non conductor for electroplating. In the preferred embodiment of theinvention, the term "non conductor" refers to a printed circuit boardbase material such as a copper clad epoxy or phenol sheet.

The term "U.K. Patent" means U.K. Patent No. 2,123,036B.

The term "catalytic metal" means a metal catalytic to the deposition ofelectroless metal and includes noble and precious metals as described inU.S. Pat. No. 3,011,920 and non noble metals catalytic to electrolessdeposition as disclosed in U.S. Pat. Nos. 3,993,799 and 3,993,491.However, where non noble metals are used as the catalytic metal, cautionmust be exercised in the use of etchants to avoid removal of thecatalytic metal sulfide conversion coating and for this reason, wherethe non noble metals are used, the process of the invention may not besuitable for formation of printed circuits using pattern platingprocedures.

Description of the Preferred Embodiments

The subject invention is suitable for manufacture of a variety ofcommercial articles where a metal deposit is desired over the surface ofa nonconductor. However, the invention is especially useful for thefabrication of printed circuit boards. For this reason, the descriptionthat follows is directed primarily to printed circuit board manufacture.

A recent trend in the printed circuit board industry is to use apermanganate conditioner in the fabrication sequence. Though desirable,it is optional. The description which follows illustrates the process ofthe subject invention without permanganate treatment though it should berecognized that permanganate treatment may be used and the advantages ofpermanganate treatment will be realized if incorporated into the processof the invention. Details of permanganate treatment can be found in U.S.Pat. No. 4,515,829, incorporated herein by reference.

In printed circuit board manufacture, the substrate commonly used is anepoxy substrate filled with glass fibers and copper clad on at least oneof its surfaces. As is known in the art, the epoxy can be substituted ormixed with other resins for specific purposes.

In the manufacture of a double-sided printed circuit board, a first stepcomprises the formation of through holes by drilling or punching or anyother method known to the art. Following formation of the holes, it isdesirable to employ the conventional steps of desmearing the holes (Step1 above) by sulfuric acid, chromic acid or plasma etching or etchback ofthe holes with chromic acid, followed by glass etching. Thereafter, theprocessing sequence of the subject invention may be employed.

Following desmearing or etchback of the holes, the circuit board basematerial is conventionally treated with a glass etch that removes glassfibers extending into the holes from the hole walls. This is followed bya solution that cleans the copper surface and conditions the hole wallto promote catalyst adsorption. Such solutions are often referred to ascleaner conditioners and typically comprise an aqueous alkalinesurfactant solution for cleaning soil and a quaternary amine tocondition the hole wall. This treatment step, by itself, is old in theart and does not constitute a part of the invention. Proprietary cleanerconditioners are commercially available and a suitable material isavailable from Shipley Company Inc. of Newton, MA and identified asCleaner Conditioner 1175.

The next step in the processing sequence is immersion of the part in acatalyst pre-dip solution. Such solutions consist of the same medium asthe catalyst solution but without the colloid. The purpose is to preventthe pH and chloride concentration of the catalyst from being altered bydragging in rinse water. As with the cleaner conditioner, the catalystpre-dip is a conventional step and does not constitute a part of thesubject invention. Proprietary catalyst pre-dip compositions arecommercially available and a suitable material is available from ShipleyCompany Inc. and identified as Cataprep® 404.

The next step in the process comprises catalysis of the surface of thenonconductor. Catalysis involves immersion of the nonconductor into anaqueous catalyst composition. The catalysts of U.S. Pat. Nos. 3,011,920and 3,874,882 are preferred catalysts for this purpose. These catalystscomprise the reduction product formed by the reduction of a noble orprecious catalytic metal by tin in acidic medium. The reduction productof palladium by tin in acidic media is the most preferred catalyticmaterial for purposes of this invention. A suitable proprietary catalystis identified as Cataposit® 44 catalyst and is available from ShipleyCompany Inc. Non noble metal catalysts are also suitable, but lesserpreferred, especially in the manufacture of printed circuit boards usingpattern plating procedures. Suitable non noble metal catalysts includecopper, nickel, cobalt, etc. and are disclosed in U.S. Pat. Nos.3,993,799 and 3,993,491 incorporated herein by reference.

The step of catalysis is accomplished by immersion of the nonconductorin the catalyst solution for a period of time ranging between 1 and 10minutes. Catalyst temperature can vary between about room temperatureand 150° F. Catalysis is required by the process of the subjectinvention. However, the procedure used to catalyze the nonconductor isin accordance with prior art procedures and does not constitute a partof the subject invention.

Following catalysis, the nonconductor is preferably contacted with asolution identified in the art as an accelerator. This material isparticularly useful when the catalyst is one formed by the reduction ofthe catalytic metal with tin. The reduction reaction forms a tin oxideprotective colloid that is believed to insulate the catalytic metal. Theaccelerator removes at least a part of the tin oxide. A suitableaccelerator is a mild acid solution such as hydrochloric acid orperchloric acid. Acceleration is accomplished by immersion of thenonconductor in an aqueous solution of the accelerator for a period oftime ranging between 1 and 5 minutes at a temperature ranging betweenabout room temperature and 150° F. Unlike the step of catalysis, thestep of acceleration is not mandatory, but is preferred. The procedureused for acceleration is in accordance with prior art procedures anddoes not constitute a part of the subject invention.

In the prior art, the next step in the process of plating a nonconductorwould be electroless metal deposition from an electroless platingsolution. In accordance with the subject invention, this step isunnecessary. Instead, the next step in the process is the formation ofthe conversion coating by the chemical conversion of the catalytic layerto a layer believed to be a chalcogenide of the catalytic layer.Chalcogenide formation occurs by contact of the catalytic layer with asolution of a chalcogen. The chalcogenide treatment solution is onecomprising a chalcogen preferably dissolved in a suitable solvent. Formetal plating operations, aqueous solutions are preferred and inaccordance with a preferred embodiment of the invention, an aqueoussolution of a water soluble chalcogen salt may be used. Sulfide is thepreferred chalcogen. Selenides and tellurides are satisfactory, butlonger plating times are required. Anhydrous oxides of some metals maybe used, but are least preferred as too great a plating time may berequired. Most preferred are alkaline earth metal sulfide salts such assodium, potassium and lithium sulfides.

In accordance with the process of the prior application, the saltcontribution was not considered to be critical and a concentration offrom 0.1 to 15 grams per liter of the salt was considered adequate witha range of from 1 to 5 grams per liter being preferred. Theconcentration range given in the prior application is accurate, but itis a discovery of the subject invention that unexpectedly, depositionrate significantly increases as the chalcogen concentration decreases.Though not wishing to be bound by theory, it is believed that thechalcogen must be present in sufficient concentration to convert thecolloid deposited over the nonconductor to a satisfactory conversioncoating, but excess chalcogen may inhibit deposition rate. Therefore, inaccordance with the subject invention, the concentration range of thechalcogen salt in solution may vary from 0.001 to 15 grams per liter ofsolution, but the preferred concentration of the chalcogen salt insolution varies between 0.001 and 2.0 grams per liter and morepreferably, ranges between 0.01 and 0.5 grams per liter.

In the prior application, when the process was used to prepare a metalclad nonconductor for electroplating, problems were from time to timeencountered using a simple sulfide salt solution. The problems werecaused by contact of sulfide ions with the metal cladding of a cladcircuit board base material. This resulted in the formation of a metalsulfide over the surface of the cladding--i.e., copper sulfide when thenonconductor is a copper clad printed circuit board base material. Thecopper sulfide formed was a dense, black layer insoluble in commonetchants but nonetheless could be removed from the copper surface towhich it was not firmly adhered using conventional procedures such asscrub cleaning. The copper sulfide surface was undesirable as itinterfered with copper to copper bonding during subsequentelectroplating processes as conventional in printed circuit boardformation.

In accordance with a preferred embodiment of the prior application,copper sulfide formation was reduced when the sulfide solution used wasone where the sulfur was covalently bonded such as the covalent bondbetween carbon and sulfur. A metal thiocarbonate solution is an exampleof a covalently bonded sulfide compound. The covalently bonded sulfideswere used in concentrations and under conditions equivalent to those setforth above for the simple sulfide solutions.

In accordance with the process of the subject invention, it has beenfound that simple salts of chalcogens are preferred to the covalentlybonded sulfide compounds of the prior application when used in lowerconcentration ranges.

In accordance with the processes disclosed herein, treatment with achalcogenide solution results in conversion of the layer of catalyticmetal to a dark brown to black, conversion coating suitable for directelectroplating. It is believed that this treatment forms a catalytic (toelectrolytic deposition) metal chalcogenide conversion coating.

Following formation of the conversion coating as described above, thenonconductor may be directly electroplated. If the nonconductor is acopper clad printed circuit base material, the copper cladding should becleaned such as, for example, by use of a sulfuric acid--hydrogenperoxide pre-etch, for example, Preposit® 746 etchant available fromShipley Company Inc. of Newton, MA. The etchant may be used at roomtemperature for a period of time ranging between 1 and 3 minutes. Itshould be noted that unexpectedly, treatment with the etchant does notresult in attack upon the conversion coating of the invention.

The next step in the process of the invention comprises electroplatingdirectly over the conversion coating avoiding the intermediate step ofelectroless metal plating. The electroplating procedure is similar tothe procedure disclosed in the above referenced U.K. Patent, but carefulcontrol of the electroplating parameters as required in the process ofthe U.K. Patent is not necessary in the process of this invention. Theelectroplating process may use electroplating solutions such as thosedisclosed in the U.K. Patent, but most commercially availableelectroplating solutions contain additives which make most commerciallyavailable electroplating solutions suitable for the process of theinvention. The preferred electroplating metals in accordance with theinvention are copper and nickel though the process is suitable forelectroplating of any desired metal. A typical electroplating solutioncomprises an aqueous acid solution of the metal desired to be platedtogether with proprietary additives from the groups of dyes,surfactants, chelating agents, brighteners, leveling agents, etc.Typical acids used in the formulation of said baths comprise those witha high ionic dissociation constant for maximum conductivity such assulfuric acid, fluoroboric acid, sulfamic acid, etc. Dyes typically usedin such baths include methylene blue, methyl violet, and otherN-heterocyclic compounds; triphenyl methane type dyes and aromaticamines, imines and diazo compounds. Suitable surfactants included insuch baths typically include non-ionic surfactants such as alkylphenoxypolyethoxyethanols, e.g., octylphenoxy, polyethoxyethanol, etc.Surfactants include wetting agents and water soluble organic compoundssuch as compounds containing multiple oxyethylene groups have been foundto be effective. A preferred group of said compounds includepolyoxyethylene polymers having from as many as 20 to 150 repeatingunits. Also included in this class of materials are block copolymers ofpolyoxyethylene and polyoxypropylene. The additives described above areadded to the solution in conventional concentrations.

The electroplating procedure is conventional. The part to be plated isused as a cathode in a conventional electroplating cell. Current densityis conventional and varies typically within a range of from 10 through40 amps per ft². Theoretically, a low initial current density should bepreferred with current density increased as an initial deposit isformed. This would be expected to prevent burn off of the thinconversion coating. However, in practice, adverse results caused by ahigh initial current density have not been observed. A preferred currentdensity range is from 15 to 30 amps per ft². The plating solution ismaintained at a temperature ranging between room temperature and about100° F. Plating is continued for a time sufficient to form a deposit ofdesired thickness. For circuit board manufacture, a desired thicknessmay range between 0.5 and 2.0 mils, typically from 1 to 1.5 mils. Aplating time of from 15 to 90 minutes would typically be required toobtain a deposit of the preferred thickness within the preferred rangeof current densities. The deposit formed by the process is uniform inthickness, free of defects and strongly bonded to the surface of thenonconductor over which it is plated. Bond strength is satisfactory towithstand solder shock testing as conventionally used in printed circuitboard manufacture.

The chemical resistance of the catalytic metal chalcogenide conversioncoating to treatment solutions permits simplified printed circuit boardmanufacturing processes impractical or inoperative in the prior art. Forexample, a pattern plating sequence, as described above, could not beused with the direct electroplating process of the U.K. Patent becausethe pretreatment steps would remove or dissolve adsorbed colloid therebymaking it impossible to first treat and image and then electroplate.This is a serious disadvantage because it severely limits the type ofcircuit board that can be fabricated using the process of the U K.Patent. The conversion coating of the subject invention remainsunaffected when contacted with treatment chemicals required for patternplating. Therefore, a pattern plating process for formation of printedcircuit boards is possible using the process of the subject invention.Such a process is illustrated in the sequence of steps described belowusing a predrilled and desmeared copper clad circuit board basematerial:

    ______________________________________                                                     Pattern Plating Sequence                                         ______________________________________                                        Step 1         Clean and condition                                            Step 2         Catalyst pre-dip                                               Step 3         Catalyze                                                       Step 4         Treat with accelerator                                         Step 5         Treat with chalcogen                                           Step 6         Acid clean copper                                                             cladding                                                       Step 7         Apply and image photo-                                                        resist                                                         Step 8         Develop photoresist image                                      Step 9         Clean and then microetch                                                      copper cladding                                                Step 10        Electroplate                                                   Step 11        Apply solder resist                                            Step 12        Remove remaining                                                              photoresist                                                    Step 13        Remove copper bared by                                                        removal of photoresist.                                        ______________________________________                                    

Step 5 above results in the formation of the chalcogenide conversioncoating of the invention. Pattern plating is possible in accordance withthe invention because the etchants and alkaline developers used todevelop the photoresist layer do not adversely effect of inactivate thesulfide conversion coating. These same materials would inactivate,desorb or dissolve the palladium tin colloidal coating used for directelectroplating in the process of the U.K. Patent.

The invention will be better understood by reference to the Exampleswhich follow where, unless stated otherwise, the substrate treated wasan epoxy copper clad circuit board base material provided with a randomarray of through holes and commercial formulations are available fromShipley Company Inc. of Newton, MA.

Examples 1 to 5

The following examples illustrate the formation of a sulfide conversioncoating over a substrate followed by electroplating.

Five circuit board substrate materials were subjected to the followingprocedure.

Step 1 Pre-clean and condition:

a. desmear the hole walls with concentrated sulfuric acid maintained ata temperature of 70° F. for 20 seconds and water rinse;

b. remove glass fibers extending into the holes by etching with anammonium bifluoride solution (1 lb./gal.) maintained at 70° F. for 4minutes and water rinse;

c. clean and condition the copper cladding and hole walls using analkaline phosphoric acid based solution containing proprietarysurfactants identified as Cleaner Conditioner 230 at 10% strength at atemperature of 140° F. for 5 minutes and water rinse.

Step 2 Catalyze:

a. immerse the substrate in an acidic sodium chloride solutionidentified as Cataprep® 404 at a temperature of 70° F. for 1 minute andwater rinse.;

b. immerse the substrate in an acidic solution of a palladium-tincolloid identified as Cataposit® 44 catalyst at a 6% strength at atemperature of 115° F. for 5 minutes and water rinse.

Step 3 Accelerate:

Immerse the substrate in an acidic accelerating solution identified asAccelerator 19 at a temperature of 70° F. for 1 minute and water rinse.

Step 4 Form conversion coating:

Immerse the substrate in a room temperature sulfide solution (as setforth in Table 1 below) for 2 minutes and water rinse. Sulfide solutionsused in the Examples are described in the following table where for eachexample, the concentration of the sulfide in solution was 10 grams perliter except in Example 5 where a neat solution was used.

                  TABLE 1                                                         ______________________________________                                        Example                                                                       Number  Sulfide        Solution pH                                                                              pH adjustor                                 ______________________________________                                        1       sodium sulfide 12-12.5    NaOH                                        2       sodium thiocarbonate                                                                         12-12.5    NaOH                                        3       sodium diethyl-                                                                              12-12.5    NaOH                                                dithiocarbamate                                                       4       sodium dithiodi-                                                                             12-12.5    NaOH                                                glycolic acid                                                         5       carbon disulfide                                                                             NA         NA                                          ______________________________________                                         Boards prepared by the preceding steps were observed for appearance. The     copper surface of the board of Example 1 had a thick dark gray film while     those of Examples 2 to 5 possessed a thinner film. The boards prepared in     accordance with Steps 1 to 4 and having a sulfide conversion coating were     electroplated by the following sequence of steps:

Step 5 Remove conversion coating from copper:

Immerse the coated board in a peroxide sulfuric etchant identified asPre-Etch 746 etchant at a temperature of 110° F. for 1 minute and waterrinse.

Step 6 Microetch the copper surfaces:

Immerse the board in a persulphate etchant (1/4 lb./gallon) identifiedas Pre-Etch 748 at 70° F. for 1 minute and water rinse.

Step 7 Electroplate:

Electroplate copper from an acid copper electroplating bath identifiedas Electroposit® 892 acid copper at a current density of 30 amps/sq. ft.and at a temperature of 70° F. for 30 minutes and water rinse.

Treatment of Example 1 boards with the peroxide etchant (step 5)resulted in large flakes of precipitate floating in the bath. The originof this material is the film on the copper surface. The film can be seenlifting off the copper surfaces when the board is immersed in the bathand large flakes of the insoluble material are seen in the bath. Whenthe boards of Examples 2 to 5 are immersed in the peroxide etchant,precipitate also appears, but to a much lesser extent. In all cases,copper plated onto the walls of the through holes and onto the coppercladding exhibit excellent bond strength.

Examples 1 and 2 constitute the most preferred embodiments of the directelectroplating process of the invention even though Example 1 requirescleaning of the heavier sulfide coating.

Examples 6 and 7 These examples demonstrate a process for panel plating.Boards were prepared following the procedures defined in steps 1 through7 above using the sodium sulfide solution of Example 1 (Example 6) andthe sodium thiocarbonate solution of Example 2 (Example 7). Followingelectrolytic deposition over the sulfide conversion coating, thefollowing steps were used:

Step 8 Apply and image photoresist:

a. dry the cleaned boards;

b. apply a dry film of Riston® 3015 photoresist (available from E. I.DuPont de Nemours and Company of Wilmington, DE) at an applicationtemperature of between 98° and 110° C. and at a speed of from 1 to 3ft/min. and wait 15 minutes following application of the film; and

c. expose the film to a source of activating energy to obtain thedesired circuit pattern at an exposure of 40 mJ/cm and then wait 15minutes.

Step 9 Develop imaged photoresist:

Place the imaged board in a spray chamber using a solution consisting of5 pounds of sodium carbonate and 1 gallon of butyl carbitol per 60gallons and develop at a temperature of 85° F. for 1 minute.

Step 10 Electroplate solder:

Immerse the substrate in a tin/lead fluoroborate plating solutionidentified as Electroposit 280 Tin/Lead at a temperature of 85° F. usinga cathode current density of between 10 and 40 amps/sq. ft. for 60minutes.

Step 11 Remove photoresist:

Spray the board with a 2% potassium hydroxide solution at a temperatureof 85° F. for 1 minute.

Step 12 Etch copper

Spray the board with an ammoniacal copper etchant at 110° F. until allof the exposed copper is removed. In both examples, copper coverage overall surfaces including through holes was excellent.

Examples 8 and 9

The following examples demonstrate a process for pattern plating in theformation of a printed circuit board using the process of the invention.

Boards were prepared following the procedures defined in steps 1 through5 of Examples 1 to 5 using the sodium sulfide solution of Example 1(Example 8) or the sodium thiocarbonate solution of Example 2 (Example9). Following formation of the sulfide conversion coating and theremoval of sulfide coating from copper surfaces, the following stepswere followed:

Step 6 Apply and image photoresist:

a. dry the cleaned boards;

b. apply a dry film of Riston® 3015 photoresist available from E. I.DuPont de Nemours and Company of Wilmington, DE at an applicationtemperature of between 98°and 110° C. and at a speed of from 1 to 3ft/min. and wait 15 minutes following application of the film; and

c. expose the photoresist film to a source of activating energy througha master to obtain the desired circuit pattern at an exposure of 40mJ/cm and wait 15 minutes.

Step 7 Develop the photoresist:

Place the substrate in a spray chamber using a solution consisting of 5pounds of sodium carbonate and 1 gallon of butyl carbitol per 60 gallonsof developer and develop at a temperature of 85° F. for 1 minute.

Step 8 Clean copper:

Immerse the sulfide coated board in Acid Cleaner 811 at 110° F. for 1minute and water rinse.

Step 9 Microetch copper surfaces:

Immerse the board in a persulphate etchant (1/4 lb./gallon) identifiedas Pre-Etch 748 at a temperature of 70° F. for 1 minute and water rinse.

Step 10 Electroplate

Electroplate copper from an acid copper electroplating bath identifiedas Electroposit® 892 acid copper at a current density of 30 amps/sq. ft.and at a temperature of 70° F. for 30 minutes and water rinse.

Step 11 Electroplate solder:

Immerse the developed board in a tin/lead fluoroborate plating solutionidentified as Electroposit 280 Tin/Lead at a temperature of 85° F. usinga cathode current density of between 10 and 40 amps/sq. ft. for 60minutes.

Step 12 Remove photoresist:

Spray the board with a 2% potassium hydroxide at a temperature of 85° F.for 1 minute.

Step 13 Etch copper surfaces:

Spray the board with an ammoniacal copper etchant at 110° F. until allof the exposed copper is removed. The above procedure produced circuitboards with good copper to copper bonds. Example 9 constitutes the mostpreferred embodiment of the invention for formation of circuit boardsusing pattern plating procedures.

Examples 10 to 11

To determine the elemental composition of the sulfide conversion coatingand the relative proportions of the elements at various stages in theprocess of the invention, an Electron Scanning Chemical Analysis (ESCA)was performed. This process comprises bombarding a surface of thecatalytic metal sulfide conversion coating with high energy electronsand observing the energies of the emitted inner-shell electrons for thevarious elements.

Six ABS coupons were treated as follows where the treatment stepsreferred to are those of Example 2:

    ______________________________________                                        Coupon No.     Treatment Steps                                                ______________________________________                                        1              1 to 3                                                         2              1 to 3 followed by step 5                                                     (Step 4 omitted)                                               3              1 to 4                                                         4              1 to 5                                                         5              1 to 5 followed by Pre-Etch                                                   748 (1/4 lb./gallon) at                                                       70° F. for 1 minute.                                    6              Same as coupon 5 followed by                                                  Lea Ronal PCM acid copper at                                                  30 amp/sq ft for 30 minutes.                                   ______________________________________                                    

Coupons 1 and 2 were set aside for visual inspection. The resultsobtained from ESCA analysis of coupons 3 through 6 are set forth in thefollowing table where palladium, tin and sulfur are in percentages andwhere carbon and oxygen have been omitted from the percentagecalculations.

    ______________________________________                                        Coupon  Percentages    Ratios                                                 Number  Pd     Sn       S    Pd/Sn   Pd/S Sn/S                                ______________________________________                                        3       34.5   10.7     54.8 3.224   0.630                                                                              0.195                               4       50.0    9.6     40.4 5.208   1.238                                                                              0.238                               5       61.9   11.9     26.2 5.202   2.363                                                                              0.454                               6       41.8    4.8     53.4 8.708   0.783                                                                              0.090                               ______________________________________                                    

Visual observation showed that coupon 1 was darkened, indicating asubstantial amount of colloidal palladium-tin catalyst had beenadsorbed. Coupon 2 was returned to its original light color afterPre-etch 746 treatment indicating that the metal colloid was easilydissolved and rinsed off the plastic. ESCA analysis of coupon 2 found notin and only two atomic percent palladium (the remainder being carbonand oxygen). After sulfide treatment of the palladium tin colloidparticle, the data established that palladium has only one chemicalenvironment (presumably palladium sulfide), the tin has two environments(possibly tin sulfide and tin oxide), and the sulfur has twoenvironments with no more than 10% of it as other species (possiblyvarious oxidative states of sulfur).

Upon immersion of coupon 6 in the electroplating bath for 20 minutes,copper was deposited outward from the metal clip holding the coupon.Plating was stopped before the coupon was completely coated and theunplated area was analyzed. As can be seen in Table II, the percentageof sulfur present remained high, indicating that the sulfide conversioncoating, which enables plating to occur, remained on the couponthroughout the rigorous electroplating process, suggesting that thesulfide conversion coating is durable.

Examples 12 and 13

This example illustrates formation of selenium and tellurium conversioncoatings and plating the same using a 2 inch by 3 inch coupon formedfrom a copper clad multilayer board with through holes drilled atselected locations as the substrate. In the examples, unless otherwisestated, where commercially available solutions are used, they are usedin accordance with the manufacturer's standard instructions for suchuse. The plating sequence comprised the following steps:

Step 1 Swell and Etch:

a. contact surface with an alkaline aqueous solution containing anorganic solvent identified as MLB Conditioner 212 at 145° F. for 5seconds and water rinse;

b. contact surface with an alkaline permanganate solution identified asMLB Promoter 213 maintained at 170° F. for 10 minutes and water rinse;

c. neutralize permanganate residues and remove glass fibers by contactwith an aqueous alkaline solution of an amine identified as MLBNeutralizer/Glass Etch 219 at a temperature of 120° F. for 5 minutes andwater rinse.

Step 2 Clean and Condition:

clean and condition the copper cladding and hole walls using an aqueouspolyamide solution identified as Cleaner Conditioner 231 at 10% strengthat a temperature of 100° F. for 5 minutes and water rinse.

Step 3 Catalysis:

a. immerse the substrate in an acidic sodium chloride solutionidentified as Cataprep® 404 at a temperature of 70° F. for 1 minute andwater rinse; and

b. immerse the substrate in an acidic solution of a palladium-tincolloid identified as Cataposit® 44 catalyst at a 6% strenqth at atemperature of 110° F. for 10 minutes and water rinse.

Step 4 Accelerate:

immerse the substrate in an acidic accelerating solution identified asAccelerator 19 at a temperature of 70° F. for 30 seconds and waterrinse.

Step 5 Form conversion coating:

immerse the substrate in a room temperature treatment solution (asdescribed below) for 45 seconds at 70° F. to form a conversion coatingand water rinse. Treatment solutions used in these Examples to form aconversion coating were prepared as follows:

Example 12--A solution is prepared by dissolving 3.3 grams of elementaltellurium in 51 mls of concentrated nitric acid and swirling theresulting slurry for several minutes. The solution so formed is slowlydiluted with distilled water until a clear green solution is obtainedwith all tellurium dissolved in the solution. This solution is dilutedwith 500 ml of distilled water and sodium hydroxide solution is added tobring the pH to between 12 and n 12.5. The solution is then diluted withdistilled water to 1 liter.

Example 13--An aqueous solution is formed by adding 1000 ppm of SeleniumStandard (1.4 grams per liter of SeO₂ from American Scientific Products)to distilled water. Aqueous sodium hydroxide is added to bring the pH tobetween 12 and 12.5.

The boards prepared in accordance with Steps 1 to 4 and having aconversion coating were electroplated by the following sequence ofsteps:

Step 6 Remove conversion coating from copper:

immerse the coated board in a peroxide sulfuric etchant identified asPre-Etch 746 etchant to which a surfactant is added at a temperature of120° F. for 1 minute and water rinse.

Step 7 Electroplate:

electroplate copper from an acid copper electroplating bath identifiedas Electroposit® 892 acid copper at a current density of 30 amps/sq. ft.and at a temperature of 70° F. until complete coverage is obtained andwater rinse. Coverage of the copper cladding and the walls of thethrough holes is complete.

I claim:
 1. A method for formation of a catalytic metal sulfideconversion coating over the surface of a nonconductor, said methodcomprising the steps of:a. treating the surface of said nonconductorwith an acid colloidal solution of a tin-noble metal electroless metalplating catalyst; and b. treating the surface of the nonconductor with asolution containing a dissolved sulfide capable of reacting with themetal plating catalyst to form a sulfide of said catalytic noble metal.2. The method of claim 1 where the nonconductor is an organic polymer.3. The method of claim 2 where the solution used to form the sulfide isa solution of an alkali or alkaline earth metal sulfide in aconcentration of from 0.1 to 15 grams per liter of solution.
 4. Themethod of claim 3 where the concentration varies from 1 to 5 grams perliter of solution.
 5. The method of claim 1 where the noble metal ispalladium.
 6. The method of claim 3 where the solution used to form thecatalytic sulfide is a solution of an aqueous soluble metal sulfidesalt.